Process of preparing polyethylene terephthalate using metal silicates as catalyst



PROCESS SF PREPARING POLYETHYLENE TER- EPHTHALATE USING METAL SILICATES AS CATALYST Mary E. Carter, Philadelphia, and John A. Price, Swarthmore, Pa., assignors to FMC Corporation, Philadelphia, Pa., a corporation of Delaware No Drawing. Filed Nov. 14, 1966, Ser. No. 593,738

Int. Cl. C08g 17/13 US. Cl. 260-75 11 Claims ABSTRACT OF THE DISCLOSURE Process of preparing polyethylene terephthalate comprising carrying out a direct esterification reaction between terephthalic acid and ethylene glycol in the presence of a metal salt of silicic acid and polycondensing the resulting product.

This invention relates to a method of preparing filament-forming linear polyestersv In particular, it relates to a method of preparing polyethylene terephthalate resin having excellent filament-forming properties.

The manufacture of filament-forming polyester resin from a dicarboxylic acid and a diol is well-known in the art. Generally, in the preparation of such polyesters, a dicarboxylic acid and glycol are first combined and subjected to a direct esterification reaction. The resulting product or prepolymer is then polycondensed at higher temperatures and under reduced pressure in the presence of a polycondensation catalyst to form the polyester resin. Various additives have been suggested heretofore for use in the first stage or esterification step to enhance the reaction. However, generally, none of these have proved entirely satisfactory since many of those known, for instance, are not capable of producing suitable prepolymers for preparing linear polyester resins having sufiiciently high molecular weights within a relatively short reaction period.

From a commercial standpoint, it is essential that a polyester resin be produced in the shortest possible time and that the desired degree of polymerization be obtained. A polyethylene terephthalate resin suitable for melt spinning into filaments should have a carboxyl content value of about or below 50 equivalents per million grams (eq./ gr. or meq./ kg.) and an intrinsic viscosity preferably not less than about 0.60 (as determined in a 60% phenol and 40% tetrachloroethane solution, wt./wt., at 30 C.). Additionally, it is essential that such filaments be substantially colorless, and possess a high degree of tenacity.

It is an object of this invention to prepare polyethylene terephthalate resin suitable for melt extrusion into nondegraded processable filaments by a direct esterification and polycondensation procedure.

Another object of the present invention is to provide an improved method for conducting the direct esterification reaction between ethylene glycol and terephthalic acid in the preparation of polyethylene terephthalate.

These and other objects are accomplished in accordance with the present invention which involves a method of preparing filament-forming polyethylene terephthalate wherein the terephthalic acid and ethylene glycol are directly esterified and the product of esterification is polycondensed in the presence of a polycondensation catalyst, the improvement comprising carrying out the direct esterification reaction in the presence of a metal salt of silicic acid or metal silicate in an amount sufiicient to improve the properties of the resulting polyester.

The silicates that are used in the direct esterification step of the present method may be suitably varied to meet 3,45l,970 Patented June 24., 1969 any requirements of reaction conditions and desired product. While the present invention is not limited to any particular metal salts of silicic acid, it hasbeen found that the preferred salts are those wherein the metal component is from Groups IA, II-A, IV-A, II-B, VII-B, and VIII of the Periodic Table (See Merck Index, sixth edition, inside front cover). For example, among the first stage or esterification additives that can be used in accordance with the present method are barium metasilicate, cadmium metasilicate, calcium metasilicate, cobalt metasilicate, lithium silicate, lithium metasilicate, lead metasilicate, manganous metasilicate, Zinc orthosilicate, nickel metasilicate, zinc metasilicate, and sodium metasilicate.

Generally, a catalytic quantity of a silicate of the present invention in the range of from about 5 10 to about 5X10 mole per mole of terephthalic acid in the subject terephthalic acid-ethylene glycol reaction mixture is used in the present direct esterification method. Higher or lower concentrations of the present catalysts can also be used. However, when concentrations less than the above are used, their effectivenessis generally reduced whereas if concentrations greater than this are used, no further improvement in the present method or desired product is generally obtained.

In general, the preparation of filament-forming polyesters of the present invention via the direct esterification reaction is carried out with a molar ratio of ethylene glycol to terephthalic acid from about 1:1 to about 15:1 but preferably from about 1.5 :1 to about 2.5:1. The first stage direct esterification step of the present method is generally carried out at temperatures ranging from about 220 C. to about 290 C. in the absence of an oxygen containing gas at atmospheric or elevated pressure for about two to four hours. For example, the reaction may be carried out in an atmosphere of nitrogen. When the direct esterification step is completed, as indicated, for example, by collection of a clear distillate, any remaining glycol is distilled off and a polycondensation catalyst is added to the esterified reaction product. The second stage or polycondensation step of the present method is generally carried out under reduced pressure within the range of from about 0.05 to 20 mm. of mercury in the absence of an oxygen containing gas at temperatures from about 260 to 325 C., for about two to six hours.

The polycondensation step of the present method is accomplished through the use of a conventional polycondensation catalyst, for example, antimony trioxide, antimony tetraoxide, antimony trisulfide, zinc acetylacetonate, and the like. The polycondensation catalyst may, if indicated, be added to the present reaction mixture before initiating the first stage or direct esterification reaction between ethylene glycol and terephthalic acid or after the reaction product thereof is formed. The polycondensation catalysts are generally employed in concentrations ranging from about 0.005 to about 0.5%, based on the total weight of the reactants.

The process of this invention may be carried out either continuously or batch-wise.

The following examples of several preferred embodiments will further serve to illustrate the present invention. All parts are by weight unless otherwise indicated.

EXAMPLES A mixture containing 84 grams (0.5 mole) of terephthalic acid, 62 grams (1.0 mole) of ethylene glycol, and 5 10 mole of a metal salt of silicic acid, or metal silicate, as listed in the following table with the exact Weight used in the above reaction mixture, was charged to a Fischer-Porter pressure assembly equipped with a nitrogen sparge tube and a distilling arm. The reactor was lowered into an oil bath maintained at 260 C. and

flushed for ten minutes with dry nitrogen. A nitrogen pressure of 60 p.s.i. was applied and a distillate of waterethylene glycol was collected. When a clear liquid, i.e. solution, was obtained, the pressure was reduced to atmospheric and the remaining excess glycol was distilled. Then, the resulting low molecular weight prepolymer was further reacted in the presence of 0.04%, based on the weight of the prepolymer, of a conventional polycondensation catalyst, e.g. antimony trioxide, antimony tetraoxide, or antimony trisulfide, under sub-atmospheric pressure of about .1 mm. of mercury for about four hours at 282 C. to form a polyester resin.

The following table sets forth conditions and results of various reactions carried out as described above.

resin wherein terephthalic acid and ethylene glycol are directly esterified and the product of esterification is polycondensed in the presence of a polycondensation catalyst, the improvement comprising carrying out the direct esterification reaction in the presence of a catalytic amount of a metal salt of silicic acid direct esterification catalytic additive wherein the metal component of said salt is from Groups IA, II-A, IVA, II-B, VII-B, or VIII of the Periodic Table (Merck index, sixth edition).

2. The method of claim 1 wherein the salt is present in an amount ranging from about l0- to about 5 X" mole per mole of terephthalic acid.

3. The method of claim 1 wherein the salt is barium metasilicate.

TABLE Weight of Prepolymcr Pol mer metal E sterl ficacarboxyl Condenearlicxyl silicate tion time, content, sation Intrinsic content, Example M nu ili t (gm.) hrs. :min. meqJkg. catalyst viscosity meq./kg.

1 3:40 316 0. 36 2 2:40 125 813203 1 0. 58 34 3 3:15 130 SbzSs 0.78 45 mo 225 50204 0.07 44 5 Cobalt metasilicate 2145 160 200 3 Sb204 0.63 4?. 6 Lithium silicate 0. 0060 2:45 280 SbgO; 0. 70 55 7. Lithium metasilioate 0.0040 2:50 260 SbzOs 0. 66 27 0. 0141 2150 are sbisi 0.71 37 0. 0000 2=45 150 S0203 1.10 27 10 0. 0008 3:00 185 512203 0. 95 21 11 0. 0142 2:50 125 S020; 0.78 23 12 Zinc oithosilicate 0. 0 2:50 95 $1020 0.60 58 1 2 hour polycondensation time.

The intrinsic viscosity of the polyester resin products of the above examples were measured in a 60% phenol and 40% tetrachloroethane solution (wt/wt.) at C.

The results shown in the above table indicate that the presence of a metal silicate during the direct esterification step in the production of polyester resin, in general, facilitates the preparation of and improves the prepolymer formed and in turn the polyester resin product. Through the use of such an additive, the direct esterification reaction time is reduced and the resulting prepolymer is, in general, characterized as being a more highly esterified product than one produced when no esterification additive is used as indicated by the carboxyl content of the prepolymer. Further, the prepolymers of the present method can be condensed to yield polyester resins which have high molecular weights as indicated by their intrinsic viscosity and which are suitable for filament-forming purposes.

The process of the present invention has been described with particular reference to polyethylene terephthalate; however, it will be obvious that the subject invention includes within its scope the preparation of other polymeric polymethylene terephthalates prepared from glycols of the series HO(CH ,OH, where n is 2 to 10 and terephthalic acid and copolyesters containing varied amounts of other suitable dicarboxylic acids such as isophthalic acid.

We claim:

1. In a method for preparing polyethylene terephthalate References Cited UNITED STATES PATENTS 3/1956 Billica et a1. 5/ 1962 Gruschke et al.

WILLIAM H. SHORT, Primary Examiner.

LOUISE P. QUAST, Assistant Examiner.

US. Cl. X.R. 

